Host-parasite arms races and rapid changes in bird egg appearance.

Coevolutionary arms races are a powerful force driving evolution, adaptation, and diversification. They can generate phenotypic polymorphisms that render it harder for a coevolving parasite or predator to exploit any one individual of a given species. In birds, egg polymorphisms should be an effective defense against mimetic brood parasites and are extreme in the African tawny-flanked prinia (Prinia subflava) and its parasite, the cuckoo finch (Anomalospiza imberbis). Here we use models of avian visual perception to analyze the appearance of prinia and cuckoo finch eggs from the same location over 40 years. We show that the two interacting populations have experienced rapid changes in egg traits. Egg colors of both species have diversified over time, expanding into avian color space as expected under negative frequency-dependent selection. Egg pattern showed signatures of both frequency-dependent and directional selection in different traits, which appeared to be evolving independently of one another. Host and parasite appear to be closely tracking one another's evolution, since parasites showed closer color mimicry of contemporaneous hosts. This correlational evidence suggests that hosts and parasites are locked in an ongoing arms race in egg appearance, driven by constant change in the selective advantage of different phenotypes, and that coevolutionary arms races can generate remarkably rapid phenotypic change.